Almost anybody who has developed a program for Microsoft Windows or a cross-platform effort involving it before is familiar with the .NET Framework. Most Windows users are even familiar with it, since they have to install it to run a huge number of programs. Back in 2014, Microsoft made the decision to make the framework open-source, with an emphasis on opening up the developer community and making the most of .NET's cross-platform capabilities. To that end, they created the .NET Foundation. Anybody with their nose in the tech world can tell you that they were practically at war with Google during that time, and to a large extent, that's still the case. Despite this, it seems that Google has been added to the fold, now part of the .NET Foundation's "Technical Steering Group", which essentially means that they will help to shape the platform's standards, uses, capabilities, and public perception.
A big part of the decision on Google's part reportedly boils down to ensuring top-class support for .NET in Google Cloud Platform. The relationship is mutually beneficial in a lot of ways that users will end up seeing, of course; Microsoft will see .NET being developed faster and more broadly with Google's big manpower and big pockets behind the project, and Google will see wider adoption of their Cloud Platform, which is, ironically enough, currently vying for supremacy against Microsoft's own Azure cloud service, both specks in the shadow of Amazon Web Services. Even more ironically, extending the capabilities of .NET will mean that Azure sees additional features, and thus wins additional market share.
Since .NET is cross-platform and is working toward being fully open-source, the possibility of seeing current Windows programs and more full-featured apps for other platforms, especially Google's Android, Chrome OS, and perhaps Andromeda in the future, is very real. While having cross-platform tools available makes porting between processor architectures easier, it's not magic; don't expect to be using AutoCAD or playing the new DOOM on your phone any time soon. Likewise, though this move is definitely one of camaraderie under Nadella and Pichai, Windows 10 users will probably still not be able to use OK Google or Google Now in the notification tray like Chrome used to have any time soon, if ever.